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Lecture 7- photosynthesis 3

H2o o2 light independent reactions chemical energy atp

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Unformatted text preview: l energy (ATP, NADPH) (C H2O) O2 Light-independent reactions Chemical energy (ATP, NADPH) CO2 Water split 08:55 + Chemical energy (C H2O) Carbon fixed 14 Gardiner Photosynthesis – What are we making? Light CO2 + H2O CHO + O2 Actually 6CO2 + 6 H2O carbon dioxide 08:55 C6H12O6 + 6O2 water glucose Gardiner oxygen 15 5 RUBISCO Reaction Gardiner 1202B 08:55 BIOLOGY • Melvin Calvin and Andy Benson discovered the pathway for glucose biosynthesis. • Reactions occur in the chloroplast stroma. • Experiment: Followed the incorporation of radioactive CO2 into biological molecules • 3-phosphoglycerate is the first, stable intermediate • Precursor is RuBP, ribulose-1,5bisphosphate. • Enzyme is RUBISCO. 16 - Most important enzyme of the biosphere - Provides the source of organic carbon molecules for most of Rubisco Gardiner 17 • Of the other 50% , 95% are light harvesting proteins Gardiner 1202B – Attaches Carbon from CO2 to ribulose bisphosphate – Most abundant protein in nature – Salad greens are 50% rubisco BIOLOGY • Occurs in stroma • Light independent • Rubisco ( ribulose bisphosphate carboxylase) 08:55 1202B 08:55 BIOLOGY • RuBP carboxylase or Rubisco • One of the largest and most complex enzymes known • 480,000 Daltons (other proteins 15,000 to 50,000 Daltons) • 30% of protein of leaf • Most abundant protein on Earth!! the worlds organisms - Rubisco accounts for about 50% of the total protein content of plant leaves - Most abundant protein on Earth - Catalyzes very slow- - > processes 3- 10 molecules of CO2 per second - Three dimensional structure - Cubed shape molecule containing 8 small subunits and 8 large subunits - Large subunit contains an active site, which has de5ined binding sites for both CO2 and RuBP - Small subunits do not have a role in catalysis but have an important regulatory role - The synthesis of RuBP requires the coordinated expression of genes in two different genomes - Large subunit is encoded by a gene of the chloroplast genome (synthesized in the stroma of the chloroplast) - Small subunit is encoded by a gene in the nucleus - After the small subunit polypeptide is synthesized in the cytosol it is imported into the chloroplast, where is associated with large subunit monomers to make the enzyme 18 - Rubisco is inef5icient at 5ixing carbon dioxide because the active site is not speci5ic to CO2 - A molecule of O2 can bind to the active site and react with RuBP - When this occurs, a two- carbon compound is produced and exported from the chloroplast and actually requires the cell to consume ATP to convert it into carbon dioxide, which is lost - Phosorespiration- - > the wasteful process because it occurs in the light and is similar to cellular respiration in that it consumes O2 and releases CO2 6 7.5a Rubisco Is an Ancient Enzyme That is Inhibited by Oxygen - Rubisco is found in a huge diversity of organisms - The atmosphere 3 billion years ago contained only trace amounts...
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